Frequency synthesizers presently find wide application in radar and/or communication systems that require generation of different frequency signals having low noise, low spurious components, a wide frequency selectable range and fast switching time. For example, radar systems utilize local oscillators to generate frequencies that are used to up-convert signals to an RF range for transmission and down-convert any received radar signals. Past approaches to frequency synthesizer design have used a signal oscillator and at least one phase lock loop for frequency generation within a limited frequency range. However, frequency bandwidth and limitations in speed for changing frequencies have made these types of indirect frequency synthesizers generally unable to meet the needs of advancing radar and communications system technologies.
In some cases, signal oscillators utilize crystal oscillators to provide specific oscillation frequencies. Use of crystal oscillators can improve system performance because crystal oscillators can be manufactured having very stable and predictable oscillation periods; hence providing low noise and low spurious oscillation frequencies. However, if the design of the radar system is changed, or if there is a change in the operational requirements of the system, which results in a change of the system operational frequencies, then a new crystal oscillator is required based on conventional system design. However, the process of producing these crystal oscillators can be very time consuming (on the order of 6 to 9 months), which can drastically delay the deployment of the system.
Furthermore, during operation, there are times during which a system has to switch between frequencies on the order of microseconds. Conventional systems can employ two crystal oscillators and switch between them as required. However, advanced systems usually require much more than two operating frequencies, therefore it is not practical to use signal oscillators that employ simple switching among crystal oscillators for systems that require 10 or more operating frequencies.